Can't dissolve certain edges
Ngons aren't the only problem which can prevent Dissolve from working, non-manifold geometry is one more. There are internal faces in your mesh left after applying Mirror modifier:



Above Select > Select All By Trait > Non-Manifold was used to select all internal faces.

Remove the faces and then dissolve the edges with either Limited Dissolve or X > Dissolve Edges.

In more general case this is one of the problems which arise if you don't delete the faces which are on the mirror plane of the modifier (if you add Subsurf these will be visible as a seam in the middle of the mesh).